This invention relates to a process for drying moist CO.sub.2 at conditions above or near the critical point (87.9.degree. F. and 1071 psia). More particularly, the present invention relates to an efficient process for drying supercritical or nearcritical CO.sub.2 to an extent materially reducing its corrosivity without the severe loss of desiccant and contamination of CO.sub.2 which happens in drying CO.sub.2 with a glycol such as triethylene glycol at conditions above or near the critical point of CO.sub.2.
Numerous desiccants and systems for drying gases are described in U.S. patents such as the following: U.S. Pat. No. 2,812,830 describes a system using triethylene glycol (TEG) and recycling partially spent TEG while removing most of the water with dry TEG. U.S. Pat. No. 3,390,511 describes a gas drying system using aqueous electrolyte desiccant adsorbed on water soluble carrier pellets such as sodium chloride pellets. U.S. Pat. No. 3,609,942 describes the drying of wet hydrocarbon gases at pressures of from about 500 to 2000 psig with ethylene glycol or other glycols. U.S. Pat. No. 3,750,369 describes a system for regenerating spent liquid desiccants such as a glycol or hygroscopic solution of electrolytes such as lithium, calcium or other chloride salts. U.S. Pat. No. 3,885,926 describes an aqueous desiccant solution of calcium chloride and magnesium nitrate which is said to be particularly suitable for dehydrating hydrocarbon gases. U.S. Pat. No. 4,235,289 describes a system for producing supercritical CO.sub.2 from a subterranean reservoir and drying it with TEG to provide dried supercritical CO.sub.2. U.S. Pat. No. 4,344,486 describes an oil recovery process for producing CO.sub.2 containing contaminants such as hydrocarbons and hydrogen sulfides from an underground reservoir, burning the mixture to form a concentrated carbon dioxide stream, compressing and dehydrating the stream with a molecular sieve or ethylene glycol water-removing arrangement and injecting the treated CO.sub.2.
Such patents contain no suggestion of any significant problem due to a glycol such as TEG remaining dissolved in the dehydrated CO.sub.2.
An article in Oil and Gas Journal, Nov. 8, 1971, page 53, describes preliminary plans for pipelining CO.sub.2 to the Sacroc unit of the Wasson field for use in oil recovery. The article describes the desirability of maintaining a pressure greater than about 1400 psig to keep the CO.sub.2 supercritical and indicates that a maximum of 50 parts per million of water in the dehydrated CO.sub.2 would adequately prevent corrosion of the pipeline. It also indicates that, in pilot tests, drying with TEG appeared satisfactory but "little experience exists in large scale dehydration of CO.sub.2 produced from natural gas wells". An Oil and Gas Journal article (Jan. 3, 1983, page 92) relating to CO.sub.2 injection for enhanced oil recovery, indicates that the gas should be dehydrated to a water dew point of about 10.degree. F. below the minimum ambient temperature and that TEG offers the best combination of ease in operation and economics since dew points obtained with it are normally within the range required for CO.sub.2 handling systems. An Oil and Gas Journal article (Mar. 21, 1983, page 160) indicates that "Corrosion by CO.sub.2 can be prevented in gas transmission lines by dehydrating the gas to less than 60% relative humidity for the coldest portion of the pipeline." and also that oil-soluble amine film-forming inhibitors can be used in CO.sub.2 -pipelines.
In summary, the prior art teachings and beliefs indicate (1) the desirability of pipelining CO.sub.2 as a supercritical single phase fluid, (2) the desirability of drying such CO.sub.2 to a non-corrosive moisture content, and (3) an expectation that triethylene glycol is the best dessicant for such a drying operation.